In a single lens reflex camera, the mirror is raised after the release operation, and the shutter is run after the mirror has been raised. In many cases, while the mirror is being raised, the wide open lens is simultaneously stopped down to a desired aperture value. If, in such a case, the mirror raising speed is not within a certain range of speeds, then the shutter may be run before the lens has stopped down to a desired aperture value, or even if the lens has been stopped down to the desired aperture value, the shutter may be run before the aperture becomes stable. This is clearly undesirable.
In order to overcome this difficulty, a system of maintaining the mirror raising speed within a predetermined range of speeds has been employed. In one example of such a system, a mirror governor mechanism for controlling the mirror raising motion with a flywheel is used. The conventional mirror governor mechanism will be described with reference to FIG. 1. As shown in FIG. 1, a mirror 1 is fixedly placed on a mirror seat 2, and a mirror supporting member 3 integral with the mirror seat 2 is rotatable about a mirror rotating shaft 4. One end portion of the mirror supporting member 3 is formed into a sector-shaped gear 3a, which is engaged with a gear 5a of a double gear 5, the number of teeth of which is smaller. The gear 5b is engaged with a gear 6a of a flywheel 6.
As the mirror 1, the mirror seat 2 and the mirror supporting member 3 are raised as one unit, the rotating motion is accelerated through the sector-shaped gear 3a and the double gear 5 which form a gear train, and therefore the gear 6a of the flywheel 6 tends to turn quickly; however, the speed of upward movement of the mirror 1 is limited because the inertial moment of the flywheel 6 is large.
In the case of a camera such as a 35 mm single lens reflex camera in which the mirror's inertial moment is relatively small, the above-described system can satisfactorily achieve its desired purpose. However, in the case of a camera such as a semi-format or 6.times.7 cm format single lens reflex camera the mirror of which is larger and heavy, the system suffers significantly from several problems. More specifically due to the significant weight of the mirror, when the posture of the camera is changed, the mirror raising speed is greatly changed. In order to compensate for this speed change, it is necessary to use a mirror governor mechanism having a gear train with a larger gear ratio and a flywheel whose inertial moment is larger. If such a mirror governor mechanism is employed, when at the end of upward movement the mirror it is stopped by contacting a contact member of foam rubber which is provided on a mirror box, the force maintaining the rotation of the flywheel is considerably large. This considerable torque of the flywheel is applied to the mirror, as a result of which the mirror bounces at the contact member. Further, when the mirror contacts the contact member, the degree of mirror shock tends to increase owing to the force of the flywheel, and an excessive force is applied to the gear train. Therefore, for instance in the case of FIG. 1, the gear 5b of the double gear 5, the number of teeth of which is larger, and the gear 6a of the flywheel 6 are liable to be damaged.